This invention relates to an image forming apparatus in which a full-color copying is performed with a photosensitive recording medium such as a microcapsule sheet, and more particularly to an image forming apparatus having a drive control unit for controlling the power supply to and the driving of elements constituting the image forming apparatus.
In a conventional image forming apparatus such as a copying machine, an original is placed between an original glass stand and an original cover which are provided at the top portion of the image forming apparatus in such a manner as to be reciprocatedly movable in a horizontal direction. The original is irradiated with light by a light source comprising a halogen lamp and a semi-cylindrical reflector while the original glass stand and the original cover are moved, thereby performing a scanning operation of the original. The light reflected from the original is directed to an exposure stand at the center portion of the image forming apparatus through various optical systems and applied to one surface of a microcapsule sheet which has been supplied on the exposure stand, whereby a latent image is formed on the microcapsule sheet. The microcapsule sheet having the latent image on one surface thereof is contacted with a developer sheet under pressure to form a visible image on the developer sheet in a pressure developing process. The developer sheet having the visible image is heat-fixed in a heat-fixing process and then is discharged to an outside of the apparatus.
The conventional image forming apparatus as described above is equipped with a power source for receiving a power from a commercial power source. The power source rectifies the power from the commercial power source to obtain a D.C. voltage (for example, 24 volts) and supplies the D.C. voltage to respective elements of the apparatus such as a light source for irradiating light to the original, a driving unit for moving the original glass stand and so on, an exposure stand heater for keeping the temperature of the exposure stand above a predetermined value, driving motors for driving sheet feed rollers, pressure-developing rollers and other rollers, a heater for keeping a heat roller of a heat-fixing unit at a constant temperature, and so on. That is, the conventional image forming apparatus has the power source for commonly supplying predetermined voltages to the elements for performing a series of coping processes. Of the above elements, the driving motor for the pressure-developing rollers in the pressure-developing unit and the heaters for the exposure stand and the heat-fixing unit provide high loads to the power source, and the driving periods thereof in the series of the copying processes are overlapped with one another. Provision of these elements having the high loads in the apparatus requires a power source having a sufficiently large capacity to the degree that all of the elements can be simultaneously driven. This requirement prevents the image forming apparatus itself from being miniaturized and the cost thereof and the power consumption from being reduced.
Further, when the elements having high loads are simultaneously driven, the operations of the elements are liable to be unstable because the power (voltage) is commonly supplied from the power source to the elements and an unstable operation of one of the elements causes the other elements to be unstably operated. For example, the fluctuation of the voltage to be applied to one of the elements causes the fluctuation of the voltages to be applied to the other elements.
The above disadvantage of the conventional image forming apparatus will be described in detail in a case where a power is commonly supplied from the power source to the light source and the heater of the heat-fixing unit.
The light source of the image forming apparatus is so designed that the amount of light of the light source is changeable to adjust light and darkness of an image to be copied. In order to adjust the amount of light of the light source, light produced by the light source is irradiated to a white reference plate, and the light reflected from the white reference plate is detected by a sensor to measure the amount of light of the reflected light. Thereafter, the voltage to be applied to the light source by the power source is adjusted so that the measured amount of light by the sensor is within a predetermined value.
On the other hand, the temperature of the heater in the heat-fixing unit is monitored and controlled by a thermistor so as to be within a predetermined range. For example, in accordance with an output signal of the thermistor, a current supply to the heater is carried out when the temperature of the heater is below a predetermined value, and is not carried out when the temperature of the heater is above a predetermined value.
Accordingly, in the conventional image forming apparatus are there two cases where an operation of adjusting the light amount of the light source and an operation of supplying the current to the heater of the heat-fixing unit are simultaneously carried out, and where only one of the two operations is carried out. That is, when the amount of light of the light source is adjusted ( the voltage to be applied to the light source by the power source is adjusted), there is two cases where the current is supplied to the heater from the power source (hereunder referred to as "current supply state") and where the current is not applied to the heater from the power source (hereunder referred to as "non current supply state"). Therefore, the voltage to be applied to the heater is fluctuated in accordance with the two states ( that is, the current supply state and the non current supply state because the voltage is commonly supplied to the light source and the heater of the heat-fixing unit. This fluctuation of the voltage to be applied to the light source causes the adjustment of the light amount of the light source to be unstable, and thus the adjustment of the light and darkness of the image to be copied is not accurately performed. In place of the common supply of the voltage to the light source and the heater of the heat-fixing unit, two power sources used separately for the light source and the heater, respectively, may be provided. However, in this case, plural electrical parts such as terminals for connecting to the power sources, a transformer and so on are required, and the construction is complicate. Accordingly, this method is not satisfied for practical use.